Pulmonary edema is an abnormal accumulation of fluid in the extravascular tissue and spaces of the lung which results in poor respiration.
Pulmonary edema represents a common problem seen in many clinical settings, and there are a number of causes of pulmonary edema such as an increase in hydrostatic pressure, increase in capillary permeability and a decrease in lymphatic clearance (Ref. 1).
The increase in hydrostatic pressure is caused by an increase in the venous pressure which usually occurs due to heart failure. The change in the permeability of the capillaries is also an important cause of pulmonary edema. If the exchange between plasma and tissue is altered in such a manner as to allow the exchange of fluid between the interstitial space and plasma more readily, then this results in an increase in fluid. The third possible cause of pulmonary edema is the decrease in lymphatic clearance. In the case where this is altered there will be an accumulation of fluid in the interstitial space. The two distinct consequences of the fluid increase are, 1) the alveoli volume will decrease in size due to the volume increase of the interstitial space and 2) a reduction in the alveoli space due to fluid accumulation.
Pulmonary edema is readily detectable by conventional chest radiographs, the present standard being X-ray. Attempts have been made to use X-rays as both a qualitative and quantitative assessment of edema (Ref. 2). The method can be performed in one of two ways, a visual inspection or a Computed Tomography (CT) of the radiograph. There are a number of small features which can be seen on plain chest radiographs. The most common is a scattered increase in lung radiographic density sometimes referred to as "clouding" (Ref. 3). This change is often difficult to recognize. CT scans involve a densitometric analysis of the radiograph (Refs. 4 and 6). However, this approach to quantitative pulmonary edema is limited. The problem with the diagnosis of pulmonary edema is the interpretation of the radiographs. The methods involve observing the physiological changes experienced pre- and post-pulmonary edema. The changes involved are increases in vessel diameters, bronchial wall thickening, and opacities associated with fluid accumulation. In the extreme case of pulmonary edema, a dense spot on the radiograph as well as physiological enlargement of the chest cavity make the identification simple. In the early stages of edema, radiographs can be misleading and interpreted in a number of manners making detection virtually impossible. Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) are also used but experience the same problem of identification (Ref. 5).
While chest radiographs are an invaluable asset to the qualitative identification of edema, they are of limited value in diagnosis of pulmonary edema in early stages of its development. Pulmonary edema is life-threatening and difficult to treat; existing radiographic methods for its diagnosis only provide effective diagnosis at an advanced stage of development.